1. Field of the Invention
This invention relates to a new electrical insulating oil composition. More particularly, the present invention relates to an electrical insulating oil composition which comprises a mixture of aromatic hydrocarbons having diphenylmethane structure and is suitable for impregnating oil-filled capacitors.
2. Description of the Prior Art
In the conventional art, PCB (polychlorobiphenyl) was used all over the world as an insulating oil for high power capacitors for electric power. PCB has a high dielectric constant, however, the use of PCB was prohibited because its toxicity was found. After that, in order to provide insulating oils having a high dielectric constant, there have been proposed insulating oils comprising a mixture of chlorinated alkyldiphenyl ether, phthalic acid esters and benzene trichloride; and esters of benzyl alcohol and fatty acids.
The oils having a high dielectric constant such as PCB were used for capacitors in which a solid insulating material of insulating paper or combined film of insulating paper and biaxially oriented polypropylene film was used. However, as the power loss of PCB and paper is large, the power loss of capacitors with these materials was large as the whole, especially at lower temperatures. For example, the loss at temperatures of +10.degree. to +20.degree. C. is approximately 0.1%, meanwhile the loss increases abruptly by ten times to 1% at temperatures of -20.degree. C. to -30.degree. C. For this reason, the generation of heat by the power loss in a capacitor cannot be disregarded and the temperature rise of +20.degree. C. to +30.degree. C. is caused to occur which depends upon the sizes of capacitors, kinds of of solid insulating materials and configurations of electrodes. As a result, even when the temperature of an insulating oil is low, for example below the pour point, the temperature is gradually raised by the internal heat generation of the capacitor. The temperature thus exceeds the pour point of the insulating oil in due course, and finally, the viscosity is lowered and the insulating oil can functions as a liquid substantially. As a result, PCB was regarded that it can be used under considerably low temperature conditions. In other words, the heat generation by power loss is essentially undesirable, however, it was exceptionally regarded desirable in the case of PCB in low temperature uses.
Meanwhile, bicyclic aromatic hydrocarbons such as 1-phenyl-1-xylylethane (PXE) and monoisopropylbiphenyl (MIPB) were proposed as the substitute for PCB. The power loss of them is small as compared with that of PCB. The loss is on the level of about 0.01% to 0.02% which is one tenth of PCB capacitor. Even at temperatures as low as -40.degree. C., the dielectric loss does not exceed 0.1%. Accordingly, the temperature rise in a capacitor owing to the power loss is generally lower than 5.degree. C. In the case of capacitors impregnated with the bicyclic aromatic hydrocarbons, the compensation by the self heat generation of power loss at lower temperatures like PCB capacitors cannot be expected.
The insulating oils of the series of the foregoing bicyclic aromatic hydrocarbons are excellent in the partial discharge characteristic as compared with PCB and the like compounds having a high dielectric constant. In addition, the former ones are excellent also in impregnating property relative to solid insulating materials such as plastic films. Accordingly, the power capacitors are mainly impregnated with them.
For the above reason, it has been eagerly desired to propose bicyclic aromatic hydrocarbons that are useful in lower temperatures with making the most of the advantages of the bicyclic aromatic hydrocarbons.
There are following conditions for the electrical insulating oils of bicyclic aromatic hydrocarbons which is suitable for impregnating foil-wound type film capacitors:
(1) The proportion of aromatic carbons in the molecule is high. The compound having aromatic hydrocarbons of a high proportion excels in hydrogen gas absorbing capacity and voltage withstanding characteristic.
(2) In order to improve the low temperature characteristics, a lower melting point is desirable.
(3) The compound must be a liquid of low viscosity even in low temperatures.
As the bicyclic aromatic hydrocarbons having a highest proportion of aromatic carbons in molecules, noncondensed bicyclic aromatic hydrocarbons having smallest numbers of 12 and 13 carbon atoms are exemplified. However, the melting points of all of these bicyclic aromatic hydrocarbons having 12 and 13 carbon atoms are high or their flash points are low. Therefore, they cannot be used as practical electrical insulating oils.
Accordingly, we cannot but select compounds from bicyclic aromatic hydrocarbons having 14 or more carbon atoms.
As a condition for an insulating oil having good low temperature characteristics, the reason for observing the viscosity at low temperatures is as follows:
If there is neither foreign substance nor defect in crystalline structure in insulating materials such as film or paper, or there is no weak deteriorated portion of the film caused by an insulating oil, the partial discharge at lower temperatures will firstly occur and the solid insulating material then suffers damages, or by the expansion of discharge, the capacitor is finally broken down.
The conditions until the beginning of partial discharge is considered as follows:
As a preliminary phenomenon, the electric potential is concentrated to the projected portions of electrode or weakened portions of solid insulating material, then gases, mainly hydrogen gas, are produced from the insulating oil surrounding such the portions. The gases are produced intensively from one portion, or they are produced in a plurality of points simultaneously. The produced gases are dissolved in the insulating oil in the initial stage and they are diffused by the difference in gas concentration or the movement of liquid dissolving gases. Meanwhile, because the bicyclic aromatic hydrocarbons generally can absorb hydrogen gas, it is considered that the absorption of gas is occurring in other portions where gas is not produced. When the quantity of produced gas exceeds the quantities to be diffused and absorbed, it exceeds the saturation level and minute bubbles are produced and finally the electric discharge is caused to occur. One of parameters for this phenomenon is the difficulty in gas generation of an insulating oil, which is considered to be closely related to the hydrogen gas absorbing capacity of the insulating oil. Another parameter is the rate of gas diffusion in the insulating oil. It is considered that the gas diffusion is caused by the combination of the phenomenon of diffusion owing to the difference in gas concentrations and the phenomenon of transfer of dissolved gas owing to the flow of liquid. Both of these two phenomena are functions of viscosity. If a temperature is the same, it is considered that a lower viscosity is advantageous because the rate of diffusion is large.
Benzyltoluenes have 14 carbon atoms and they are one group of the bicyclic aromatic hydrocarbons which are highest in aromaticity. In addition, with regard to the benzyltoluenes, the viscosity of their isomer mixture is less than 200 cSt at -50.degree. C. in a supercooled condition before crystals are separated out. Taking the low temperature of -50.degree. C. into consideration, its viscosity is very low. In general, the viscosity at the pour point or its vicinity is tens of thousands to a hundred thousands cSt. Therefore, it can be said that the viscosities of benzyltoluenes at low temperatures are very low and they have good low temperature characteristics as electrical insulating oils.
With regard to benzyltoluenes, examples of o-benzyltoluene, p-benzyltoluene and the mixtures of these benzyltoluenes and dibenzyltoluene are disclosed in Japanese Patent Publication No. 55-5689. Furthermore, disclosed in U.S. Pat. No. 4,523,044 are examples of electrical insulating oils comprising oligomer compositions obtained by reacting benzyl chloride with toluene in the presence of iron chloride catalyst, that is, the mixture of substantially benzyltoluenes and dibenzyltoluenes.
Furthermore, an electrical insulating oil consisting of a mixture of benzyltoluene and dibenzyltoluene has been commercialized as "JARYLEC C-100" (trademark) by Prodelec Co. in France.
As disclosed in the foregoing reference, these benzyltoluenes are prepared from benzyl chloride and toluene by Friedel-Crafts reaction using iron chloride catalyst which is high in o-, p-orientation. Accordingly, the main components are o-benzyltoluene and p-benzyltoluene and the quantity of m-benzyltoluene is small. It is considered that the dibenzyltoluene was by-produced in the preparation of the benzyltoluenes.
In order to improve the low temperature characteristic of an insulating oil, the melting point thereof is desirably low. According to references, the melting points of the position isomers of benzyltoluenes are as follows:
TABLE 1 ______________________________________ Position Isomers of Benzyltoluenes Melting Point Heat of Fusion Compound (.degree.C.) (cal/mol) ______________________________________ o-Benzyltoluene +6.6 5000 m-Benzyltoluene -27.8 4700 p-Benzyltoluene +4.6 4900 ______________________________________
In view of the above Table 1, the melting points of o-isomer and p-isomer themselves are high, so that they cannot be used singly even in the Temperate Zone. m-Benzyltoluene is a component of a small quantity (less than 10%) in the foregoing U.S. Pat. No. 4,523,044 and in JARYLEC C-100 (trademark). It has a lowest melting point among these position isomers, however, its melting point is higher than the pour point that is provided in a common standard (e.g. Japanese Industrial Standards, JIS) for the mineral insulating oils.
That is, as described above, the viscosities at low temperatures of benzyltoluenes are low, however, their melting points are not always satisfactory.
In order to solve such a problem, dibenzyltoluene produced as a by-product is mixed with benzyltoluene in the description of U.S. Pat. No. 4,523,044.
For example, in the foregoing JARYLEC C-100 which is considered to be the same as the description of the above patent specification, about 20% by weight of dibenzyltoluene is added to benzyltoluenes. The depression of freezing point (the point at which crystals are separated out) is proportional to the number of moles of added substance, known as the phenomenon of freezing point depression. Accordingly, 20% by weight of dibenzyltoluene corresponds to 14.3% by molar concentration. At this molar concentration, the depression of the point of separating out is only 6.degree. to 8.degree. C. In other words, the effect of depressing the temperature of separating out is not so large for its weight as added because the molecular weight of dibenzyltoluene is large. In addition, the advantage of low viscosity of benzyltoluene is impaired by the addition of dibenzyltoluene because the viscosity of dibenzyltoluene is higher than that of benzyltoluene.
Even when the separating out of crystals is apparently restrained by the supercooling, it is rather not desirable because viscosity becomes higher at low temperatures.
This fact was confirmed by tracing the disclosure of the foregoing U.S. Pat. No. 4,523,044 with the experiment of the present inventors as follows:
In the like manner as the example in the above reference, benzyl chloride and toluene were reacted in the presence of a catalyst of FeCl.sub.3 ; and benzyltoluene and dibenzyltoluene were obtained by distillation. These benzyltoluene and dibenzyltoluene in a weight ratio of 80:20 were mixed together. The contents of isomers of the benzyltoluene in the obtained mixture were o-isomer: 39.1 wt %, m-isomer: 5.4 wt % and p-isomer, 35.5 wt %, which were almost coincident with the analytical values of the above JARYLEC C-100 of o-isomer: 36.2 wt %, m-isomer: 5.9 wt % and p-isomer: 37.8 wt %.
The above synthesized benzyltoluene, the mixture of benzyltoluene and dibenzyltoluene, and JARYLEC C-100 were respectively put in stoppered test tubes. They were left to stand in a temperature-programmable refrigerator to observe the state of separating out of crystals. One temperature cycle was 12 hours between -40.degree. C. and -50.degree. C.
According to the results of this test, crystals were separated out after 1 to 3 days and the whole was solidified in the case of only benzyltoluene. In the case of the mixture of benzyltoluene/dibenzyltoluene and JARYLEC C-100, the separating out of crystals began after 4 to 7 days and crystals grew gradually, and after 2 weeks, crystals were observed on almost all the walls of test tubes. That is, the viscosity was increased by the addition of dibenzyltoluene to maintain the supercooled state long, and the time period for crystallizing out was prolonged. Accordingly, even though crystals were separated out finally, the crystallizing out was retarded by the addition of dibenzyltoluene.
However, because the viscosity is definitely raised by the addition of dibenzyltoluene, it is adverse to the object of the present invention to obtain an electrical insulating oil which has a low viscosity even at low temperatures.
Therefore, the method of the foregoing U.S. Pat. No. 4,523,044 cannot provide substantial improvement in benzyltoluene.